Degenerative audio amplifier



July 14, 1942. H. BoucKE DEGENERATIVE AUDIO AMPLIFIER Filed Dec.l l2, 1946 .Il juda 'Patented July 1942 DEGENERATIVE lAUI-)IO AMPLIFIER Heinz Boucke, Berlin, Germany,V assigner to Telefunken Gesellschaft fr Drahtlose Telegraphie m. b. H., Berlin, Germany, a corporation o1' Germany Application December 12, 1940, Serial No. 369,817 In Germany December 13, 1939l a claims. i (ci. 179 171) This invention relates to an audio frequency amplifier, and more particularly to the audio-network of a receiver with selective negative feedback known in the art.

According to the invention a selective negative current feedback designed to attenuate the higher audio frequencies is combined in a tube, furnished with a single grid, with a capacitative shunt-adapted to attenuate the high audio fre- `quencies in the plate circuit of the same tube.

What is essential is that a negative current feedback rather than a voltage feedback is used.

The basic features of the invention are known in the art. However, what happens by using the invention is a steeper decline, or droop, of the frequency response curve for the high audio fre.

quencies; infact, the same is more marked than where the same steps are used in' distinct stages of the amplifier.

Inthe drawing:

Figs. 1 and 2 show respectively different embodiments of the invention,

Fig. 3 shows various frequency response curve relating to the invention,

Fig. 4 shows a further modification.

Fig. 1 shows a single-grid tube 2 provided with negative current feedback to input transformer I. The cathode resistance 3a with parallel condenser in the customary way results in a grid biasing voltage. Across the cathode resistance 3b is taken off the negative feedback potential by way of a frequency-dependent voltage divider comprising resistance 4 and condenser 5, the said negative feedback potential increasing with increase of frequency. Condenser 8 serves as an alternating current by-pass of the anode source of current supply. Across the resistance 'l is taken off the amplified audio potential. Condenser 6, as known in the prior art, isso proportioned that its reactance, for the highest audio frequency to be reproduced, is roughly as high as, or higher than, the internal resistance of the tube (without negative feedback). Resistance as also known in the'earlier art, is chosen substantially higher. Now, since for the frequencies subject to negative feedback the internalfre- -sistance of the tube appearing enlarged 'as a result of negative current feedback is several times higher than the original internal resistance, the

, output potentials inside the band vof frequencies subject to negative feedback (that is, inside the range of the higher frequencies) are attenuated more markedly than in the absence of negative regeneration. What is involved is a multiplicative shown in Fig. 2.

hand, and the change in internal resistance occasioned thereby, on the other hand, upon the output potential. As a consequence the frequency response curves exhibit a far steeper decline 'of their slopes.

So far as the audio part of a receiver is concerned, it is particularly advantageous to include for negative feedback a parallel resonant circuit tuned to 9 kilocycles (kc.) in the cathode lead, with the'resistance of the winding resulting preferably in the grid biasing potential. This is Coil I0 and' capacitor Il together form the parallel .resonant circuit tuned 'to 9kc.

Curve l in Fig. 3 shows the frequency response curve obtained in the absenceof the negative regeneration, and in the absence of condenser 6. By adding the condenser 6, in the absence of negative regeneration, there is found a curve l' 'based upon negative feedback, for if there were no additional effect, then, because of the logarithmic scale, the distance of curves 2 and 2', at resonance frequency, would have to be the same y as the distance between I and I'.

It has been ascertained thatin Fig.l 2 by variations of inductance I0 bandwidth regulation vwithin wide limits is obtainable, such regulation becoming particularly favorable because of the steepslope of. the frequency-response curve re,- sulting from the step here disclosed. To insure a change of the band-Width from around 2 kc. to about 9 kc. it is necessary to alter the inductance roughly at a ratio such as 1:25. In practical tests made with a triode and a plate resistance 1 of about 200,000 ohms, and a parallel condenser 6 of 1000 micro-microfarad:., favorable limiting values for the inductance I0 were found to be around 0.04 to 2 henry. Change of the inductance is possible by switching if the coil has taps. However, this incidentally results also in a change in'thegrid biasing voltage. Hence, it is of more practical advantage to tune the coil by change of the permeability. y

Fig. inshows one such example. The windin U-shaped core made of similar material, it is possiblel to alter the air path of the flux lines and thereby the effective permeability to a. large Y extent. By transmitting the tuning force to thev influence of the negative feedback, on the onel said U.shaped part by the intermediary of a cam nally, by the aid of a resistance I5 connected in parallel relation to the' circuit |0--II the damping may be varied, and thereby the degree of the negative feedback as a function of the fr equency can be regulated. y

Particularly advantageous has been found the scheme to carry an audio band-width variation into effect in a way as shown in Fig. 2, by causing opposite variation of the inductance and of the capacity of the resonant circuit simultaneously, say by means of a joint shaft, so that the resonance frequency stays constant. The lower in this scheme the inductance becomes the broader will become the transmitted frequency band, and the steeper, however, the slope because of decrease of the damping. This is shown by curves 3 and 3 (the latter covering the use of a condenser 6) compared with curves 2 and 2', respectively. This band-width variation in comparison to regulation by variation of the inductance only offers the merit that there is lno parallel shift of the slope; in fact, the slope, in the presence of the broadest band-width, is made additionally broader. For this reason even for adjustment to quite a broad band-width marked suppression of the 9 kc. note is insured. Also inthe case of approximatestabilization of the resonance frequency, double regulation is advantageous. Even if the inductance value that has been adjusted is high, the capacity value may be made so lhigh that the resonance frequency lies around 5000 to '7000 cycles, as shown by curve 4 in Fig. 3.

What is claimed is:

1, In an amplifier provided' with a tube having input and output circuits, means impressing currents of a wide frequency range on the input circuit, means common to the output and input circuits for providing degeneration of the higher frequencies of said range, said common means comprising reactances of opposite sign resonated to a high frequency of said range, means for varying the magnitudes of the reactance's in opposite senses thereby to control the degree of degeneration, and additional means in said output circuit for attenuating the said higher frequencies thereby to provide a highly augmented attenuation of the higher frequencies.

2. In an audio frequency amplifier, a tube having input and output electrodes,'an audio input circuit coupled to the input electrodes, an audio output circuit coupled to the output electrodes,

1 a tuned circuit, common to the input and output electrodes of said tube,v resonated to a frequency located in the upper portion of the audio range whereby said input circuit is degenerated for audio frequencies in said upper portion, means in said output circuit for augmenting the attenuation of said upper portion frequencies, and means for varying the reactances of said tuned circuit in opposed senses thereby to adjust the degree of said degeneration without changing said resonant frequency.

3. In an audio transmission network having input and output terminals, a resonant circuit, including capacity and inductance, common to the output and input terminals for providing a degenerative audio voltage feedback path, said circuit being tuned to an audio frequency lying in the upper end of the audio range thereby to render said path degenerative for the higher audio frequencies, means across the output terminals for augmenting the attenuation of said higher audio frequencies, and means for varying the magnitude of each of said inductance and capacity in opposite senses thereby to provide a Wide regulation over the band-width of the degenerated frequencies.

HEINZ BOUCKE 

